The requirements to unlock it are:
- Get clear markers for 25 different songs from the pop'n 18 category.
- Once you've done that, play pop'n 18 songs for your 1st and 2nd stages, and LOVELY KUNOICHI POP will be available at the final stage.

From Jubeat Ripples, and it's Seiya Murai and ALT so it fits Pop'n Music perfectly. Think fast ethnic sound like Kung-Fu Empire with computer voicebox vocals.

The unlock method from BEMANIwiki appears to be:
Select the character Kagetora from Pop'n Music 9 (雪の華, Taiga Drama). On the first two songs, play Polaris (Galaxive Rock) and a song with A.I. in the genre (A.I. Techno, A.I. Obachan, A.I. Kids). The song will then appear on the final stage.

Play each of the following songs at least once: くさびかたびら☆下克上！〜暗躍の魅惑MIX (Lovely Kunoichi Pop), 隅田川夏恋歌 (A.I. Date Pop), 登山者達 (Tozan, Pop'n Music 11), さようならこんにちは (Oh-Misoka, Pop'n Music 12 いろは), and 突然ゴルゴンゾーラ (Kaijuu, Pop'n Music 17 THE MOVIE). On a subsequent playthrough during the first or second song, make the number 155 appear in either the score or judgement counts (must be 155 exactly, not a number with 155 in it). The song will then appear on the final stage.

Combo was also a possibility, but smbu over on BMS already confirmed that combo doesn't work.